1. Field of the Invention
The invention relates generally to the field evaluating the integrity of bonds that adhere wellbore casing to a wellbore. More specifically, the present invention relates to a method and apparatus of producing and detecting acoustic forces within a wellbore casing to evaluate the integrity of the casing.
2. Description of Related Art
As illustrated in FIG. 1 wellbores typically comprise casing 8 set within the wellbore 5, where the casing 8 is bonded to the wellbore by adding cement 9 within the annulus formed between the outer diameter of the casing 8 and the inner diameter of the wellbore 5. The cement bond not only adheres to the casing 8 within the wellbore 5, but also serves to isolate adjacent zones (e.g. Z1 and Z2) within an earth formation 18. Isolating adjacent zones can be important when one of the zones contains oil or gas and the other zone includes a non-hydrocarbon fluid such as water. Should the cement 9 surrounding the casing 8 be defective and fail to provide isolation of the adjacent zones, water or other undesirable fluid can migrate into the hydrocarbon producing zone thus diluting or contaminating the hydrocarbons within the producing zone, and increasing production costs, delaying production or inhibiting resource recovery.
To detect possible defective cement bonds, downhole tools 14 have been developed for analyzing the integrity of the cement 9 bonding the casing 8 to the wellbore 5. These downhole tools 14 are lowered into the wellbore 5 by wireline 10 in combination with a pulley 12 and typically include transducers 16 disposed on their outer surface formed to be acoustically coupled to the fluid in the borehole. These transducers 16 are generally capable of emitting acoustic waves into the casing 8 and recording the amplitude of the acoustic waves as they travel, or propagate, across the casing 8. Characteristics of the cement bond, such as its efficacy, integrity and adherence to the casing, can be determined by analyzing characteristics of the acoustic wave such as attenuation. Typically the transducers 16 are piezoelectric devices having a piezoelectric crystal that converts electrical energy into mechanical vibrations or oscillations transmitting acoustic wave to the casing 8. Piezoelectric devices typically couple to a casing 8 through a coupling medium found in the wellbore. Coupling mediums include liquids that are typically found in wellbores. When coupling mediums are present between the piezoelectric device and the casing 8, they can communicate the mechanical vibrations from the piezoelectric device to the casing 8. However, lower density fluids such as gas or air and high viscosity fluids such as some drilling mud may not provide adequate coupling between a piezoelectric device and the casing 8. Furthermore, the presence of sludge, scale, or other like matter on the inner circumference of the casing 8 can detrimentally affect the efficacy of a bond log acquired with a piezoelectric device. Thus for piezoelectric devices to provide meaningful bond log results, they must cleanly contact the inner surface of the casing 8 or be employed in wellbores, or wellbore zones, having liquid within the casing 8. Another drawback faced when employing piezoelectric devices for use in bond logging operations involves the limitation of variant waveforms produced by these devices. Fluids required to couple the wave from the transducer to the casing only conduct compressional waves, thus limiting the wave types that can be induced in or received from the casing. A great deal of information is derivable from variant acoustical waveforms that could be used in evaluating casing, casing bonds, and possibly even conditions in the formation 18. Therefore, there exists a need to conduct bond logging operations without the presence of a particular couplant. A need exists for a bond logging device capable of emitting and propagating into wellbore casing numerous types of waveforms, and recording the waveforms.
Electromagnetic-acoustic transducers (EMATs) have been used in non-destructive testing. An EMAT acts through the following physical principles. When a wire is placed near the surface of an electrically conducting object and is driven by a current at the desired ultrasonic frequency, eddy currents are induced in a near surface region of the object. If a static magnetic field is also present, these eddy currents experience Lorentz forces. These forces cause an acoustic excitation in the object. In a reciprocal use, an electric signal will be generated in the wire as a result of acoustic excitation in a metal placed close to a permanent magnet. Attenuation and/or reflection of the acoustic waves bear information on the defects and surroundings of the object. An EMAT is typically designed to producing a single waveform, such as shear horizontal waves (SH) or Lamb waves.
Various EMAT design configurations have been proposed. U.S. Pat. No. 4,296,486 to Vasile discloses an EMAT including a source of magnetic flux for establishing a static magnetic field, an electrical conductor for conducting an alternating current in the static magnetic field, and an electrically conductive nonmagnetic shield disposes between the source of magnetic flux and the conductor. U.S. Pat. No. 7,024,935 to Paige et al. discloses an EMAT including a magnetic unit arranged to be moved relative to the material under test to magnetize a surface layer of the material, and an electrical winding supplied by an alternating current source, the magnetic unit and the electric winding, in use, being applied in sequence to the material under test whereby the electrical winding is positioned adjacent the material subsequent to magnetization thereof by the magnetic unit, the alternating magnetic flux created by the winding interacting with the remanent magnetization of the material to create ultrasonic vibration of the material.
If different excitation modes are used to characterize the object, then an array of two or more transducers is generally used. The total attraction force between the object and the array of transducers may dramatically complicate mechanics related to placing and moving the array with respect to the object.
Therefore, there exists a need for a device and method to perform acoustic testing in a borehole casing that reduces the amount of placing and moving of the transducers with respect to the object being examined.